Synchronous compensator plant

ABSTRACT

The magnetic circuit of synchronous compensator plant is included in an electric machine which is directly connected to a high supply voltage of 20-800 kV, preferably higher than 36 kV. The electric machine is provided with solid insulation and its winding(s) is/are built up of a cable ( 6 ) intended for high voltage comprising one or more current-carrying conductors ( 31 ) with a number of strands ( 36 ) surrounded by at least one outer and one inner semiconducting layer ( 34, 32 ) and intermediate insulating layers ( 33 ). The outer semiconducting layer ( 34 ) is at earth potential. The phases of the winding are Y-connected, and the Y-point may be insulated and protected from over-voltage by means of surge arresters, or else the Y-point is earthed via a suppression filter. A procedure is used in the manufacture of a synchronous compensator for such plant, in which the cable used is threaded into the openings in the core for the magnetic circuit of the synchronous compensator.

TECHNICAL FIELD

The present invention relates to electric machines intended forconnection to distribution or transmission networks, hereinafter termedpower networks. More specifically the invention relates to synchronouscompensator plants for the above purpose.

BACKGROUND ART

Reactive power is present in all electric power systems that transferalternating current. Many loads consume not only active power but alsoreactive power. Transmission and distribution of electric power per seentails reactive losses as a result of series inductances intransformers, overhead lines and cables. Overhead lines and cables alsoproduce reactive power as a result of capacitive connections betweenphases and between phases and earth potential.

At stationary operation of an alternating current system, active powerproduction and consumption must be in agreement in order to obtainnominal frequency. An equally strong coupling exists between reactivepower balance and voltages in the electric power network. If reactivepower consumption and production are not balanced in a suitable manner,the consequence may be unacceptable voltage levels in parts of theelectric power network. An excess of reactive power in one area leads tohigh voltages, whereas a deficiency leads to low voltages.

Contrary to active power balance at a nominal frequencies, which iscontrolled solely with the aid of the active power starter of thegenerator, a suitable reactive power balance is obtained with the aid ofboth controllable excitation of synchronous generators and of othercomponents spread out in the system. Examples of such (phasecompensation) components are shunt reactors, shunt capacitors,synchronous compensators and SVCs (Static Var. Compensators).

The location of these phase compensation components in the electricpower network affects not only the voltage in various parts of theelectric power network, but also the losses in the electric powernetwork since the transfer of reactive power, like the transfer ofactive power, gives rise to losses and thus heating. It is consequentlydesirable to place phase compensation components so that losses areminimized and the voltage in all parts of the electric power network isacceptable.

The shunt reactor and shunt capacitor are usually permanently connectedor connected via a mechanical breaker mechanism to the electric powernetwork. In other words, the reactive power consumed/produced by thesecomponents is not continuously controllable. The reactive powerproduced/consumed by the synchronous compensator and the SVC, on theother hand, is continuously controllable. These two components areconsequently used if there is a demand for high-performance voltagecontrol.

The following is a brief description of the technology for phasecompensation with the aid of synchronous compensator and SVC.

A synchronous compensator is in principle a synchronous motor running atno load, i.e. it takes active power from the electric power networkequivalent to the machine losses.

The rotor shaft of a synchronous compensator is usually horizontal andthe rotor generally has six or eight salient poles. The rotor is usuallydimensioned thermally so that the synchronous compensator, inover-excited state, can producr approximately 100% of the apparent powerthe stator is thermally dimensioned for (rated output) in the form ofreactive power. In under-excited state, when the synchronous compensatorconsumes reactive power, it consumes approximately 60% of the ratedoutput (standard value, depending on how the machine is dimensioned).This gives a control area of approximately 160% of rated output overwhich the reactive power consumption/production can be continuouslycontrolled. If the machine has salient poles with relatively littlereactance in transverse direction, and is provided with excitationequipment enabling both positive and negative excitation, more reactivepower can be consumed than the 60% of rated output stated above, withoutthe machine exceeding the stability limit. Modern synchronouscompensators are normally equipped with fast excitation systems,preferably a thyristor-controlled static exciter where the directcurrent is supplied to the rotor via slip rings. This solution enablesboth positive and negative supply as above.

The magnetic circuits in a synchronous compensator usually comprise alaminated core, e.g. of sheet steel with a welded construction. Toprovide ventilation and cooling the core is often divided into stackswith radial and/or axial ventilation ducts. For large machines thelaminations are punched out in segments which are attached to the frameof the machine, the laminated core being held together by pressurefingers and pressure rings. The winding of the magnetic circuit isdisposed in slots in the core, the slots generally having a crosssection in the shape of a rectangle or trapezium.

In multi-phase electric machines the windings are made as either singleor double layer windings. With single layer windings there is only onecoil side per slot, whereas with double layer windings there are twocoil sides per slot. By coil side is meant one or more conductorscombined vertically or horizontally and provided with a common coilinsulation, i.e. an insulation designed to withstand the rated voltageof the machine to earth.

Double-layer windings are generally made as diamond windings whereassingle layer windings in the present context can be made as diamond orflat windings. Only one (possibly two) coil width exists in diamondwindings whereas flat windings are made as concentric windings, i.e.with widely varying coil width. By coil width is meant the distance inarc dimension between two coil sides pertaining to the same coil.

Normally all large machines are made with double-layer winding and coilsof the same size. Each coil is placed with one side in one layer and theother side in the other layer. This means that all coils cross eachother in the coil end. If there are more than two layers these crossingscomplicate the winding work and the coil end is less satisfactory.

It is considered that coils for rotating machines can be manufacturedwith good results up to a voltage range of 10-20 kV.

A synchronous compensator has considerable short-duration overloadcapacity. In situations when electromechanical oscillations occur in thepower system the synchronous compensator can briefly supply reactivepower up to twice the rated output. The synchronous compensator also hasa more long-lasting overload capacity and is often able to supply 10 to20% more than rated output for up to 30 minutes.

Synchronous compensators exist in sizes from a few MVA to hundreds ofMVA. The losses for a synchronous compensator cooled by hydrogen gasamount to approximately 10 W/kvar, whereas the corresponding figure forair-cooled synchronous compensators is approximately 20 W/kvar.

Synchronous compensators were preferably installed in the receiving endof long racial transmission lines and in important nodes in maskedelectric power networks With long transmission lines, particularly inareas with little local generation. The synchronous compensator is alsoused to increase the short-circuit power in the vicinity of HVDCinverter stations.

The synchronous compensator is most often connected to points in theelectric power network where the voltage is substantially higher thanthe synchronous compensator is designed for. This means that, besidesthe synchronous compensator, the synchronous compensator plant generallyincludes a step-up transformer, a busbar system between synchronouscompensator and transformer, a generator breaker between synchronouscompensator and transformer, and a line breaker between transformer andelectric power network, see the single-line diagram in FIG. 1.

In recent years SVCs have to a great extent replaced synchronouscompensators in new installations because of their advantagesparticularly with regard to cost, but also in certain applicationsbecause of technical advantages.

The SVC concept (Static Var. Compensator) is today the leading conceptfor reactive power compensation and, as well as in many cases replacingthe synchronous compensator in the transmission network, it also hasindustrial applications in connection with electric arc furnaces. SVCsare static in the sense that, contrary to synchronous compensators, theyhave no movable or rotating main components.

SVC technology, is based on rapid breakers built up of semi-conductors,thyristors. A thyristor can switch from isolator to conductor in a fewmillionths of a second. Capacitors and reactors can be connected ordisconnected with negligible delay with the aid of thyristor bridges. Bycombining these two components reactive power can be steplessly eithersupplied or extracted. Capacitor banks with different reactive powerenable the supplied reactive power to be controlled in steps.

A SVC plant consists of both capacitor banks and reactors and since thethyristors generate harmonics, the plant also includes harmonic filters.Besides control equipment, a Transformer is also required between thecompensation equipmentand the network in order to obtain optimalcompensation from the size and cost point of view. SVC plant isavailable in size from a Feel MVA up to 650 MVA, with nominal voltagesup to 765 kV.

Various SVC plan types exist, named after how the capacitors andreactors are combined. Two usual elements that may be included are TSCor TCR. TSC is a thyristor-controlled reactive power-producing capacitorand TCR is a thyristor-controlled reactive power-consuming reactor. Ausual type is a combination of these elements, TSC/TCR.

The magnitude of the losses depends much on which type of plant the SVCbelongs to, e.g. a FC/TCR type (FC means that the capacitor is fixed)has considerably greater losses than a TSC/TCR. The losses for thelatter type are approximately comparable with the losses for asynchronous compensator.

It should be evident from the above summary of the phase compensationtechnology that this can be divided into two principal concepts, namelysynchronous compensation and SVC.

These concepts have different strengths and weaknesses. Compared withthe synchronous compensator, the SVC has the main advantage of beingcheaper. However, it also permits somewhat faster control which may bean advantage in certain applications.

The drawbacks of the SVC as compared with the synchronous compensatorinclude:

it has no overload capacity. In operation at its capacitive limit theSVC becomes in principle a capacitor, i.e. if the voltage drops then thereactive power production drops with the square of the voltage. If thepurpose of the phase compensation is to enable transfer of power overlong distances the lack of overload capacity means that, in order toavoid stability problems, a higher rated output must be chosen if SVCplant is selected than if synchronous compensator plant is selected.

it requires filters if it includes a TCR.

it does not have a rotating mass with internal voltage source. This isan advantage with the synchronous compensator, particularly in thevicinity of HVDC transmission.

The present invention relates to a new synchronous compensator plant.

Rotating electric machines have started to be used, for instance, forproducing/consuming reactive power with the object of achieving phasecompensation in a network.

The following is a brief description of this technology, i.e. phasecompensation by means of synchronous compensators and other conventionaltechnology for compensating reactive power.

Reactive power should be compensated locally at the consumption point inorder to avoid reactive power being transferred to the network andgiving rise to losses. The shunt reactor, shunt capacitors, synchronouscompensator and SVC represent different ways of compensating for theneed for reactive power in transmission and sub-transmission networks.

A synchronous compensator is in principle a synchronous motor running inneutral, i.e. it takes active power from the network, corresponding tothe losses of the machine. The machine can be under-excited orover-excited in order to consume or produce reactive power,respectively. Its production/consumption of reactive power can becontinuously regulated.

In over-excited state the synchronous compensator has a relatively largeshort-term overload capacity of 10-20% for up to 30 minutes. Inunder-excited state, when the machine consumes reactive power, it cannormally consume approximately 60% of rated output (standard valuedepending on how the machine is dimensioned). This gives a control areaof approximately 160% of rated output.

If the machine has salient poles with relatively little reactance intransverse direction and is provided with excitation plant enablingnegative excitation, it is possible for more reactive power to beconsumed than the above-stated 60% of rated output, without the machineexceeding the stability limit. Modern synchronous compensators arenormally equipped with rapid excitation systems, preferably athyristor-controlled static exciter in which the direct current issupplied to the rotor via slip rings. This solution also permitsnegative excitation in accordance with the above.

Synchronous compensators are used today primarily to generate andconsume reactive power in the transmission network in connection withHVDC inverter stations because of the ability of the synchronouscompensator to increase the short-circuiting capacity, which the SVClacks. In recent years the SVC has replaced the synchronous compensatorin new installations because of its advantages as regards cost andconstruction.

The present invention relates to the first-mentioned concept, i.e.synchronous compensation.

DESCRIPTION OF THE INVENTION

Against this background, one object of the invention is to provide abetter synchronous compensator plant than is possible with knowntechnology, by reducing the number of electrical components necessarywhen it is to be connected to high-voltage networks, including those ata voltage level or 36 kV and above.

Thanks to the fact that the winding(s) in the rotating electric machinein the synchronous compensator plant is/are-manufactured with thisspecial solid insulation, a voltage level can be achieved for themachine which is far above the limits a conventional machine of thistype can be practically or financially constructed for. The voltagelevel may reach any level applicable in power networks for distributionand transmission. The advantage is thus achieved that the synchronouscompensator can be connected directly to such networks withoutintermediate connection of a step-up transformer.

Elimination of the transformer per se entails great savings in cost,weight and space, but also has other decisive advantages over aconvention synchronous compensator plant.

The efficiency of the plant is increased and the losses are avoided thatare incurred by the transformer's consumption of reactive power and theresultant turning of the phase angle. This has a positive effect asregards the static and dynamic stability margins of the system.Furthermore, a convention transformer contains oil, which entails a firerisk. This is eliminated in a plant according to the invention, and therequirement for various types of fire-precautions is reduced. Many otherelectrical coupling components and protective equipment are alsoreduced. This gives reduced plant costs and less need for service andmaintenance.

These and other advantages result in a synchronous compensator plantbeing considerably smaller and less expensive than a conventional plant,and that the operating economy is radically improved thanks to lessmaintenance and smaller losses.

Thanks to these advantages a synchronous compensator plant according tothe invention will contribute to this concept being financiallycompetitive with the SVC concept (see above) and even offering costbenefits in comparison with this.

The fact that the invention makes the synchronous compensator conceptcompetitive in comparison with the SVC concept therefore enables areturn to the use of synchronous compensator plants. The drawbacksassociated with SVC compensation are thus no longer relevant. Thecomplicated, bulky banks of capacitors and reactors in a SVC plant areone such drawback. Another big drawback with SVC technology is itsstatic compensation which does not give the same stability as thatobtained by the inertia obtained in a rotating electric machine with itsrotating e.m.f. as regards both voltage and phase angle. A synchronouscompensator is therefore better able to adjust to temporary interferencein the network and to fluctuations in the phase angle. The thyristorsthat control a SVC plant are also sensitive to displacement of the phaseangle. A plant according to the invention also enables the problem ofharmonics to be solved.

The synchronous compensator plant according to the invention thusenables the advantages of synchronous compensator technology over SVCtechnology to be exploited so that a more efficient and stablecompensation is obtained at a cost superior to this from the point ofview of both plant investment and operation.

The plant according to the invention is small, inexpensive, efficientand reliable, both in comparison with a conventional synchronouscompensator and a SVC.

Another object of the invention is to satisfy the need for fast,continuously controllable reactive power which is directly connected tosub-transmission or transmission level in order to manage the systemstability and/or dependence on rotating mass and the electro-motiveforce in the vicinity of HVDC transmission. The plants shall be able tosupply anything from a few MVA up to thousands of MVA.

The advantage gained by satisfying said objects is the avoidance of theintermediate transformer, the reactance of which otherwise consumesreactive power. This also enables the avoidance of traditionalhigh-power breakers. Advantages are also obtained as regards networkquality since there is rotating compensation. With a plant according tothe invention the overload capacity is also increased, which With theinvention may be +100%. The synchronous compensator according to theinvention may be given higher overload capacity in over-excitedopera;ion than conventional synchronous compensators, both as regardsshort-during and long-duration overload capacity. This is primarilybecause the time constants for heating the stator are large withelectric insulation of the stator winding according to the invention.However, the thermal dimensioning of the rotor must be such that it doesnot limit the possibilities or exploiting this overload capacity. Thisenables the use of a smaller machine. The control region may be longerthan with existing technology.

To accomplish this the magnetic circuit in the electric machine includedin the synchronous compensator plant is formed with threaded permanentinsulating cable with included earth. The invention also relates to aprocedure for manufacturing such a magnetic circuit.

The major and essential difference between known technology and theembodiment according to the invention is thus that this is achieved withan electric machine provided with solid insulation, the magneticcircuit(s) of the winding(s) being arranged to be directly connected viabreakers and isolators to a high supply voltage of between 20 and 800kV, preferably higher than 36 kV. The magnetic circuit thus comprises alaminated core having a winding consisting of a threaded cable with oneor more permanently insulated conductors having a semiconducting layerboth at the conductor and outside the insulation, the outersemiconducting layer being connected to earth potential.

To solve the problems arising with direct connection of electricmachines to all types of high-voltage power networks, a machine in theplant according to the invention has a number of features as mentionedabove, which differ distinctly from known technology. Additionalfeatures and further embodiments are defined in the dependent claims andare discussed in the following.

Such features mentioned above and other essential characteristics of thesynchronous compensator plant and the electric machine according to theinvention included therein, include the following:

The winding of the magnetic circuit is produced from a cable having oneor more permanently insulated conductors with a semiconducting layer atboth conductor and sheath. Some typical conductors of this type are PEXcable or a cable with EP rubber insulation which, however, for thepresent purpose are further developed both as regards the strands in theconductor and the nature of the outer sheath. PEX=crosslinkedpolyethylene (XLPE). EP=ethylene propylene.

Cables with, circular cross section are preferred, but cables with someother cross section may be used in order to obtain better packingdensity, for instance.

Such a cable allows the laminated core to be designee according to theinvention in a new and optimal way as regards slots and teeth.

The winding is preferably manufactured with insulation in steps for bestutilization of the laminated core.

The winding is preferably manufactured as a multi-layered, concentriccable winding, thus enabling the number of coil-end intersections to bereduced.

The slot design is suited to the cross section of the winding cable sothat the slots are in the form of a number of cylindrical openingsrunning axially and/or radially outside each other and having an openwaist running between the layers of the stator winding.

The design of the slots is adjusted to the relevant cable cross sectionand to the stepped insulation of the winding. The stepped insulationallows the magnetic core to have substantially constant tooth width,irrespective of the radial extension.

The above-mentioned further development as regards the strands entailsthe winding conductors consisting of a number of impacted strata/layers,i.e. insulated strands that from the point of view of an electricmachine, are not necessarily correctly transposed, uninsulated and/orinsulated from each other.

The above-mentioned further development as regards the outer sheathentails that at suitable points along the length of the conductor, theouter sheath is cut off, each cut partial length being connecteddirectly to earth potential.

The use of a cable of the type described above allows the entire lengthof the outer sheath of the winding, as well as other parts of the plant,to be kept at earth potential. An important advantage is that theelectric field is close to zero within the coil-end region outside theouter semiconducting layer. With earth potential on the outer sheath theelectric field need not be controlled. This means that no fieldconcentrations will occur either in the core, in the coil-end regions orin the transition between them.

The mixture of insulated and/or uninsulated impacted strands, ortransposed strands, results in low stray losses.

The cable for high voltage used in the magnetic circuit winding isconstructed or an inner core/conductor with a plurality of strands, atleast two semiconducting layers, the innermost being surrounded by aninsulating layer, which is in turn surrounded by an outer semiconductinglayer having an outer diameter in the order or 20-250 mm and a conductorarea in the order of 30-3000 mm².

According to a particularly preferred embodiment of the invention, atleast two of these layers, preferably all three, have the samecoefficient of thermal expansion. The decisive benefit is thus achievedthat defects, cracks or the like are avoided at thermal movement in thewinding.

The invention also relates to a procedure for manufacturing the magneticcircuit for the electric machine included in the synchronous compensatorplant. The procedure entails the winding being placed in the slots bythreading the cable through the cylindrical openings in the slots.

Since the insulation system, suitably permanent, is designed so thatfrom the thermal and electrical point of view it is dimensioned for over36 kV, the plant can be connected to high-voltage power networks withoutany intermediate step-up transformer, thereby achieving the advantagesreferred to above.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in more detail in the following detaileddescription of a preferred embodiment of the construction of themagnetic circuit of the electrical machine in the synchronouscompensator plant, with reference to the accompanying drawings in which

FIG. 1 shows a single line diagram of the invented synchronouscompensator plant.

FIG. 2 shows a schematic axial end view of a sector of the stator in anelectric machine in the synchronous compensator plant according to theinvention,

FIG. 3 shows an end view, step-stripped, of a cable used in the windingof the stator according to FIG. 2, and

FIG. 4 is a schematic illustration of a three-phase synchronouscompensator plant in accordance with the present invention.

DESCRIPTION OF A PREFERRED EMBODIMENT:

FIG. 1 shows a single line diagram of the synchronous compensator plantaccording to a preferred embodiment of the invention, where the machineis arranged for direct connection to the power network, without anystep-up transformer, at two different voltage levels.

In the schematic axial view through a sector of the stator 1 accordingto FIG. 2, pertaining to the electric machine included in thesynchronous compensator plant, the rotor 2 of the machine is alsoindicated. The stator 1 is composed in conventional manner of alaminated core 1′. FIG. 1 shows a sector of the machine corresponding toone pole pitch. From a yoke part 3 of the core situated radiallyoutermost, a number of teeth 4 extend radially in towards the rotor 2and are separated by slots 5 in which the stator winding is arranged.Cables 6 forming this stator winding, are high-voltage cables which maybe of substantially the same type as those used for power distribution,i.e. PEX cables. One difference is that the outer,mechanically-protective sheath, and the metal screen normallysurrounding such power distribution cables are eliminated so that thecable for the present application comprises only the conductor and atleast one semiconducting layer on each side of an insulating layer.Thus, the semiconducting layer which is sensitive to mechanical damagelies naked on the surface of the cable.

The cables 6 are illustrated schematically in FIG. 2, only theconducting central part of each cable part or coil side being drawn in.As can be seen, each slot 5 has varying cross section with alternatingwide parts 7 and narrow parts 8. The wide parts 7 are substantiallycircular and surround the cabling, the waist parts between these formingnarrow parts 8. The waist parts serve to radially fix the position ofeach cable. The cross section of the slot 5 also narrows radiallyinwards. This is because the voltage on the cable parts is lower thecloser to the radially inner part of the stator 1 they are situate.Slimmer cabling can therefore be used there, whereas coarser cabling isnecessary further out. In the example illustrated, cables of threedifferent dimensions are used, arranged in three correspondinglydimensioned sections 51, 52, 53 of slots 5. An auxiliary power winding 9is arranged outermost.

The various dimensioned sections of the slots 51, 52 and 53 correspondto several different voltage levels respectively, namely higher levelVI, medium level VII, and lower level VIII. The thickness of theinsulation layer 33 may be sized to correspond to the slots. The firstthickness 33I corresponds to the higher voltage VI; a lesser thickness33II corresponds to the mid level voltage VII; and a narrower thickness33III corresponds to the lower voltage level VIII. Typically, the lowervoltage level of the cable is coupled to the Y-point.

The cable 6 in FIG. 2 is illustrated with a fixed circularcross-section. However, the cross-section of the cable may be made tocorrespond to either; the stepwise change in slot size shown in FIG. 1.Alternatively, the cross section of the cable may decease graduallyinstead of stepwise. A continuously decreasing cross-section is notshown, as its configuration would be readily apparent to one of skill inthe art.

Further as illustrated in FIG. 3, the conductive strands 36 are formedof a plurality of insulated conductive strands 36, as shown, and atleast one uninsulated strand 36A which contacts the inner semiconductinglayer 32.

FIG. 3 shows a step-wise stripped end view of a high-voltage cable foruse in an electric machine according to the present invention. Thehigh-voltage cable 6 comprises one or more conductors 31, each of whichcomprises a number of strands 36 which together give a circular crosssection of copper (Cu), for instance. These conductors 31 are arrangedin the middle of the high-voltage cable 6 and in the shown embodimenteach is surrounded by a part insulation 35. However, it is feasible forthe part insulation 35 to be omitted on one of the four conductors 31.The number of conductors 31 need not, of course, be restricted to four,but may be more or less. The conductors 31 are together surrounded by afirst semiconducting layer 32. Around this first semiconducting layer 32is an insulating layer 33, e.g. PEX insulation, which is in turnsurrounded by a second semiconducting layer 34. Thus the concept“high-voltage cable” in this application need not include any metallicscreen or outer sheath of the type that normally surrounds such a cablefor power distribution.

In accordance with the present invention, the synchronous compensationplant of the invention provides quadrature-axis synchronous reactancewhich is considerably less than the direct-axis synchronous reactance.In other words, out of phase synchronous reactance is reduced.

FIG. 4 illustrates an arrangement of the invention employing three phasecompensation. According to the invention, an exciter 40, which may be apositive or negative exciter, is coupled to the phases 42 a, 42 b and 42c of a rotating machine. The phases 42 are Y connected having a neutralpoint 44 which is connected to ground 46 via a suppression filter 48. Asurge arrester 50 may be coupled in parallel with the suppression filter48 as shown. A cooling means 52 employing either gas or liquid workingfluid 54 may be provided in heat exchange relation with the phases 42 ofthe arrangement illustrated.

What is claimed is:
 1. A synchronous compensator plant comprising atleast one rotating electric machine including at least one flexiblewinding, wherein the winding comprises a conductor and an insulationsystem surrounding the conductor including at least one semiconductinglayer forming an equipotential surface around the conductor and a solidinsulation layer wherein the current carrying conductor comprises aplurality of insulated conductive strands, and at least one uninsulatedconductive strand.
 2. The plant as claimed in claim 1, wherein at leastone of the layers and the solid insulation form a monolithic structurehaving substantially the same coefficient of thermal expansion.
 3. Theplant as claimed in claim 1, wherein the winding comprises a highvoltage cable.
 4. The plant as claimed in claim 3, wherein the at leastone semiconducting layer comprises an inner semiconducting layer is inelectrical contact with and at substantially the same potential as theconductor.
 5. The plant as claimed in claim 3, wherein at least two ofsaid layers form a monolithic structure and have substantially the samecoefficient of thermal expansion.
 6. The plant as claimed in claim 3,wherein the cable with solid insulation intended for high voltage have aconductor area of about between 30 and 3000 mm2 and have an outer cablediameter of about between 20 and
 250. 7. The plant as claimed in claim1, wherein said at least one semiconducting layer comprises an outersemiconducting layer connected to a selected potential.
 8. The plant asclaimed in claim 7, wherein the selected potential is earth potential.9. The plant as claimed in claim 1, wherein the winding comprises acable and the at least one semiconducting layer includes an innersemiconducting layer and an outermost semiconducting layer beingarranged around each conductor, and an insulating layer of solidinsulation being arranged between the inner semiconducting layer and theoutermost semiconducting layer.
 10. The plant as claimed in claim 1,wherein the machine has a magnetic circuit including a cooled statoroperative at earth potential.
 11. The plant as claimed in claim 10,wherein the electrical machine comprises a generator including a rotor.12. The plant as claimed in claim 11, wherein the machine is connectableto a local power supply for starting said machine.
 13. The plant asclaimed in claim 11, wherein the machine has two or more poles.
 14. Theplant as claimed in claim 13, wherein the rotor and the stator are sodimensioned that at nominal voltage, nominal power factor andoverexcited operation, the thermally based current limits of stator androtor are exceeded approximately simultaneously.
 15. The plant asclaimed in claim 14, wherein it has 100% overload capacity at nominalvoltage, nominal power factor and at over-excited operation.
 16. Theplant as claimed in claim 13, wherein the rotor and the stator are sodimensioned that at nominal voltage, nominal power factor andover-excited operation, the thermally based stator current limit isexceeded before the thermally based rotor current limit has beenexceeded.
 17. The plant as claimed in claim 1, wherein the electricalmachine includes a magnetic circuit comprising a stator having a centralaxis and at least one slot and a stator winding located in the slot,said slot having a number of cylindrical openings each having a centralaxis parallel with the central axis of the stator and being disposed inthe slot radially adjacent each other, each cylindrical opening having asubstantially circular cross section and being separated by narrow waistparts therebetween.
 18. The plant as claimed in claim 17, wherein themachine comprises a generator having a rotor and the stator including ayoke and the circular cross section of the substantially cylindricalopenings for the stator winding has a decreasing radius seen from theyoke towards the rotor.
 19. The plant as claimed in claim 17, whereinthe stator winding has three phases and the phases of said statorwinding are Y-connected.
 20. The plant as claimed in claim 19, whereinthe stator winding includes a Y-point insulated from earth potential orconnected to earth potential via a high-ohmic impedance and protectedfrom over-voltages by means of surge arresters.
 21. The plant as claimedin claim 19, wherein the Y-point of the stator winding is earthed via asuppression filter of third harmonic type, which suppression filter isdesigned to greatly reduce or eliminate third harmonic currents in theelectric machine and for limiting voltages and currents in the event offaults in the plant.
 22. The plant as claimed in claim 21, wherein thesuppression filter is protected from over-voltages by means of surgearresters, the latter being connected in parallel with the suppressionfilter.
 23. The plant as claimed in claim 1, wherein the at least onerotating electric machine has a high voltage side and a Y-point, andwherein the insulation seen insulation system has a thickness whichdecreases from the high voltage side towards the Y-point.
 24. The plantas claimed in claim 23, wherein the gradual decrease in the insulationthickness is stepwise or continuous.
 25. The plant as claimed in claim1, wherein the quadrature-axis synchronous reactance is considerablyless than the direct-axis synchronous reactance.
 26. The plant asclaimed in claim 25, wherein the machine is includes an excitationsystem for enabling both positive and negative excitation.
 27. The plantas claimed in claim 1, comprising stator and rotor circuits and coolingmeans therefor in which the coolant is in liquid and/or gaseous form.28. The plant as claimed in claim 1, wherein the machine is arranged forconnection to several different voltage levels.
 29. The plant as claimedin claim 1, wherein the machine is connected to the power networkwithout any step-up transformer.
 30. The plant as claimed in claim 1,wherein the winding of the machine is arranged for self-regulating fieldcontrol.
 31. The synchronous compensator plant of claim 1, wherein theinsulation system has thermal and electrical properties, which permit avoltage level in the machine exceeding 36 kV.
 32. A synchronouscompensator plant including a rotating high voltage electric machinecomprising a stator; a rotor and a flexible winding, wherein saidwinding comprises a cable including at least one current-carryingconductor including a plurality of insulated strands and a lesserplurality of uninsulated strands and a cover surrounding the conductorin electrical contact therewith, including an inner layer surroundingthe conductor and being in electrical contact therewith; and aninsulating layer surrounding the inner layer; and an outersemiconducting layer surrounding the insulating layer, said cableforming at least one uninterrupted turn in the corresponding winding ofsaid machine.
 33. The synchronous compensator plant of claim 32, whereinthe cover comprises an insulating layer surrounding the conductor and anouter layer surrounding the insulating layer, said outer layer having aconductivity for establishing an equipotential surface around theconductor.
 34. The synchronous compensator plant of claim 32, whereinthe cover is formed of a plurality of layers including an insulatinglayer and wherein said plurality of layers are joined together to form amonolithic structure and being substantially free of cracks and defects.35. The synchronous compensator plant of claim 32, wherein the cover isin electrical contact with the conductor.
 36. The synchronouscompensator plant of claim 35, wherein the layers of the cover havesubstantially the same temperature coefficient of expansion.
 37. Thesynchronous compensator plant of claim 32, wherein the machine isoperable at 100% overload for two hours.
 38. The synchronous compensatorplant of claim 32, wherein the cable is operable free of sensible endwinding loss.
 39. The synchronous compensator plant of claim 32, whereinthe winding is operable free of partial discharge and field control. 40.The synchronous compensator plant of claim 32, wherein the windingcomprises multiple uninterrupted turns.
 41. The synchronous compensatorplant of claim 32, wherein the cover is flexible.
 42. The sychronouscompensator plant comprising at least one rotating electric machineincluding at least one flexible winding, wherein the winding comprises acurrent carrying conductor and an insulation system surrounding theconductor including at least one semiconducting layer forming anequipotential surface around the conductor and a solid insulation layer,and wherein the machine is arranged for connection to several differentvoltage levels wherein the current carrying conductor comprises aplurality of insulated conductive strands, and at least one uninsulatedconductive strand.